Fix footnote labels appearing out-of-order (#536)

[Crozzers]

This PR follows up on a comment on #536 that shows that the footnote labels are appearing out-of-order, even though the footnote links themselves are accurate. This bug was due to the out-of-order processing of some markdown elements and the order of insertion into the footnote_ids list.

Example:

one [^1]

- two [^2]


[^1]: one
[^2]: two

The footnote label is inserted as each footnote is processed. However, the list is processed first, which means its footnote is processed firstthat it is given the label of 1 mistakenly.

This PR fixes that by instead inserting a placeholder here, and then postprocessing the entire text top-to-bottom, which fixes the labelling.

[nicholasserra]

Thank you!